It has been proposed that a variety of medical conditions can be treated by introducing an insertable or implantable medical device having a coating for release of a biologically active material. For example, various types of drug-coated stents have been proposed for localized delivery of drugs to a body lumen. See U.S. Pat. No. 6,099,562 to Ding et al.
However, the coatings for a medical device can exhibit problems of cracking especially when the device is exposed to harsh conditions, such as low temperatures and/or mechanical deformations. For example, a self-expanding stent must be contracted and loaded into a delivery sheath before delivering into a patient's body. To contract a self-expanding stent made of a shape-memory alloy, it must be chilled to be thermally induced into the Martenstic phase, in which the shape-memory alloy can be plastically deformed. In practice, the self-expanding stent is chilled to about −80° to −100° C. and then warmed to about −60° to −20° C. when it is contracted. However, the processing temperature about −60° to −20° C. is usually the same as or lower than the glass transition temperatures of many polymers. Therefore, when chilled to these temperatures, a polymer coating on the stent is in a condition like glass and particularly vulnerable to stress-cracking when the device is processed.
The risk of cracking the coating is particularly high in certain parts of the coated stent, such as the apex regions of a zigzag strut configuration where the surface of the strut is greatly deformed by contraction of the stent as shown in FIGS. 1, 1a and 2. FIG. 1 shows a schematic view of a portion of a stent 10 having struts 11 in its expanded state. The apex regions of the zigzag strut configuration 12 are magnified in FIG. 1a. FIG. 2 shows a schematic view of the same apex regions 12 when the stent is in its contracted state and the cracks 13 in the coating that may occur at the apex regions. Cracks in the coating are undesirable because they can cause the coating to flake or separate from the coated surface of the device while the coated medical device is inside the body of a patient. Such separated or loose pieces of coating can cause emboli. Hence, there is a need for a coated medical device wherein a risk of cracks in the coating is reduced.
Furthermore, when a medical device such as a stent is delivered to the implantation site, the coated surface of the medical device is often covered by a sheath to prevent the coating from being removed before the medical device is inserted and appropriately located inside the body. Also, if the coated surface of the medical device is self-expanding, a sheath is used to contract the portion so that the device can be inserted, such as in the case of a self-expanding stent. However, the sheath is likely to contact the coating located on the outermost portion of the coated surface. The coating material at such outermost portion may adhere to the sheath. When the sheath is withdrawn, the adhered coating may be torn or removed from the coated device. Therefore, there is a need for a coated medical device that avoids such undesired tearing of the coating.
Also, the conventional methods for coating medical devices require encapsulating the device or coating entire surfaces of the device. However, in many medical devices, not all of the surfaces or the entirety of the surfaces of the medical device need to be coated. For instance, in medical devices having a tubular portion, such as a vascular stent, the inner surface of the tubular portion does not have to be coated with a coating containing a biologically active material that is used to treat only the body lumen wall that contacts the outer surface of the stent. This is because the inner surface of the stent does not come in contact with body-lumen wall and does not apply the biologically active material to the body-lumen wall. When all the surfaces of a medical device such as a stent, including surfaces that are not directly in contact with the body tissue of a patient, are coated with a composition comprising a biologically active material, more biologically active material is used than is needed. Thus, the patient may receive unnecessary exposure to the material. Also, manufacturing costs for the medical device may be needlessly increased by including unnecessary amounts of the biologically active material in the medical device.
Moreover, if the medical device is an expandable stent, the coating on the sides of the struts may adhere to each other when the stent is placed in its contracted state. When the stent is expanded, the adhered coating may be removed from the struts. In addition, if the medical device is a balloon expandable stent, the coating on the inner surface of the stent has higher risk of damage because it directly contacts the balloon and is pressed by a balloon. Such damage is undesirable because the damaged coating may separate from the device while the device is inserted in a patient. Accordingly, there is a need for a method that can coat only the outer surface of a medical device or the surface that directly contacts the body tissue to be treated.